<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        *{
            padding: 0;
            bottom: 0;
        }
        body{
            height: 100vh;
            width: 100vw;
            display: flex;
           justify-content: center;
           align-items: center;
        }
        .box{
            width: 100px;
            height: 100px;
            background-image:radial-gradient( circle at top left,transparent 20% , blue 20%);
            background-position: 50% 50%;
        }
    </style>
</head>
<body>
    <div class="box"></div>
</body>
</html>